Inelastic deformation of rocks under uniaxial loading of compresion reflects the stresses that have been previously applied to the rocks. This effects of the previous stresses on the inelastic strain is called the rock property of stress memory. The absolute values of in situ stresses are suggested to be estimated from boring core samples by the method of deformation rate analysis(DRA) which has been developed on the basis of the property of the stress memory. We call the memory of the absolute values of in situ stress as in situ stress memory. In situ stress memory is however an unexpected property from the established knowledge on the deformation of rocks. We carried out this study for the purposes as follows ; 1) to confirm that the stresses estimated from core samples by DRA are regarded as the in situ stresses at the places where the samples were retrieved, 2) to propose a hypothesis for the explanation of in situ stress memory and 3) to prove the validity of the hypothesis. We compared the vertical stresses estimated by DRA on boring core samples up to 842 m depth with the calculated overburden pressures. a) We confirmed by the comparisom that rocks have in situ stress memory. b) We proposed the hypothesis that stress field in rocks has been uniformed at the in situ stresses which are almost constant for a long time and theoretically proved that the in situ stress memory is explianed by the hypothesis. The hypothesis predicts that the compliance of rock specimens decreases to the minimum at an applied stress nearly equal to the constant stress that has been being applied to the specimen for a long time. c) We experimentally found the decrease in the compliance for the rock specimens which have been subjected to a constant stress for a time more than 3 days.